3. DC Machines

Chapter 2 DC Machines

DC Machines

DC Machines are motors that convert dc electric energy to mechanicalenergy and generators that convert mechanical energy to dc electrical energy

Construction

Stator of dc machines

Construction

Rotor of dc machines

Construction

dc machines with housing

Part of the dc machines Stator (Field): does not move, the outer frame of the machines and

its made from ferromagnetic materials.

Rotor (Armature): free to move, the inner part of the machine and its made from ferromagnetic materials.

Field winding: is wound on the stator poles to produce magnetic field (flux) in the air gap.

Armature winding: is composed of coils placed in the armature slots.

Commutator: is composed of copper bars, insulated from each other. The armature winding is connected to the commutator.

Brush: is placed against the commutator surface. Brush is used to connect the armature winding to external circuit through commutator.

Introduction to DC Motors Earliest power systems is dc systems. 1890s ac power systems were winning

out over dc power systems. Why dc motors so common, when dc

power systems were fairly rare? Reasons:-◦ Vehicle – has a dc power system, it make

sense to consider using dc motor.◦ Provide the desired speed control.◦ There are still some applications where dc

motor are preferred.

Speed Regulation (SR) DC Motor are often compared by their speed regulation.

, ,

,

, ,

,

SR= 100%

SR= 100%

m nl m fl

m fl

m nl m fl

m fl

n nn

ω ωω−

×

−×

A positive speed regulation means a motor’s speed drops with increasing load.

A negative speed regulation means a motor’s speed increase with increasing load.

Types of dc motors

The separately excited dc motor The shunt dc motor The series dc motor The compounded dc motor The permanent-magnet dc motor

The Equivalent Circuit of a DC Motor

A mE Kφω=

ind AK Iτ φ=

Internal generated voltage

Induced torque developed

The Magnetization Curve

mmf = F FN I

Separately Excited DC Motors

Shunt DC Motors

The Terminal Characteristic of a Shunt DC Motor

( )2T A

m indV RK K

ω τφ φ

= −

It is important to realize in order for the speed of the motor to vary linearly with torque, the terminal voltage supplied by the dc power source is assumed to be constant.

Motor’s Speed

A terminal characteristic of a machine is a plot of the machine’s output versus each other. For a motor, the output quantities are shaft torque and speed.

The Terminal Characteristic of a Shunt DC Motor If terminal voltage is not constant, then

the voltage variations will affect the shape of the torque-speed curve.

Another effect internal to the motor that can also affect the shape of the torque-speed curve is armature reaction. If a motor has armature reaction, then as its load increase, the flux-weakening effect reduce its flux.

The Terminal Characteristic of a Shunt DC Motor

If a motor has compensating winding,of course there will be no flux-weakeningproblems in the machine, and flux in the machines will be constant.

Nonlinear Analysis of a Shunt DC Motor

net ARF FN I= −F F

Magnetomotive force due to armature reaction (AR),

The equivalent field current of a shunt dc motor is given by,

* ARF F

F

I IN

= −F

Induced Voltage,

'A mE K nφ=

Internal generated voltage is related to speed by,

0 0

mA

A

nEE n

=

Speed Control of Shunt DC Motors

Common Method

1. Adjusting the field resistance FR

2. Adjusting the terminal voltage

Less Common Method

3. Inserting a resistor in series with the armature circuit

Speed Control of Shunt DC Motors1. Changing the field resistance

Speed Control of Shunt DC Motors2. Changing the armature voltage

Speed Control of Shunt DC Motors2. Changing the armature voltage

Speed Control of Shunt DC Motors3. Insert a resistor in series with the armature circuit

The Series DC Motor

ind AK Iτ φ=

The Terminal Characteristic of a Series DC Motor

Speed Control of DC Series Motors

Unlike with the shunt dc motor, there are only one efficient way to change the speed of a series dc motor. That method is to change the terminal voltage of the motor. If the terminal voltage is increase, the first term in Equation torque-speed relationship is increase, resulting in a higher speed for any given torque.

1 A STm

ind

R RVKcKc

ωτ

+= −

The Compounded DC Motor

( )T A A A SV E I R R= + +

A L FI I I= −

TF

F

VIR

=

The Torque-Speed Characteristic of a Cumulatively Compounded DC MotorThe torque-speed full-load rating

The Torque-Speed Characteristic of a Cumulatively Compounded DC MotorThe torque-speed no-load rating

The Torque-Speed Characteristic of a Differentially Compounded DC Motor

DC Motor Efficiency Calculations

Copper losses Brush drop losses Mechanical losses Core losses Stray losses

Introduction to DC Generators

Separately excited generator Shunt generator Series generator Cumulatively compounded generator Differentially compounded generator

* There is no real difference between a generator and a motor except forthe direction of power flow.

Power Flow Diagram for DC Machines

Equivalent Circuit DC Generator

Voltage Regulation

nl fl

fl

V VVR= 100%V−

×

The Separately Excited Generator

Terminal Characteristic Separately Excited Generator

With compensating windings

Without compensating windings

Control of Terminal Voltage1. Change the speed of rotation.

2. Change the field current.

, FF

F

A m

T A A A

VIR

E K

V E I R

φ

φ ω

= ↑↓

= ↑

= ↑ −

A m

T A A A

E K

V E I R

φω= ↑

= ↑ −

Effect of Decrease in Field Resistance on the Output Voltage

The Shunt DC Generator

Terminal Characteristic Shunt DC Generator

The Series DC Generator

Terminal Characteristic Series DC Generator

The Cumulatively Compounded DC Generator

Terminal Characteristic Cumulatively Compounded DC Generator

Thank Yousaufiahmad@unimap.edu.my